Residual categories in a classification are labelled Not elsewhere classified (nec), Not elsewhere included (nei), Not further defined (nfd), Other or Miscellaneous.

These categories are necessary because, although in a classification meaningful categories are created through the application of certain criteria, not all observations can be classified into a homogeneous group, or the size of the observations does not allow them to be separately identified. For example, in the classification of languages, the minor group 'Chinese' is composed of six distinct languages and one residual category:

The residual category is needed because the six distinct languages do not encompass all the known Chinese languages. The remainder of observations which can be classified as 'Chinese languages' are grouped together in 'Chinese languages, nec'.

Supplementary codes (often called dump codes) are used to process inadequately described responses. Not further defined codes (sometimes called undefined codes) are used to process incomplete, non-specific or imprecise responses which cannot be coded to the most detailed level of a classification, but which nevertheless, contain enough information to allow them to be coded to a higher level of the classification structure. For example, country of birth responses relating to places which cannot be identified as lying within the boundaries of a country separately identified in the Standard Australian Classification of Countries (SACC), but which lie wholly within the boundaries of one of the classification's Minor Groups, are coded to that Minor Group.

It is important to note the distinction between Not elsewhere classified categories and Not further defined codes. NEC categories are a formal part of a classification's structure, designed to make a classification complete and exhaustive of all observations in scope. Adequately described, specific responses are coded to nec categories in instances where a suitable substantive category is not included in the classification. As explained above, nfd codes are designed to facilitate processing by allowing inadequately described or non-specific responses to be coded to a broader level of the classification rather than be lost altogether. NFD codes are not a formal part of the classification.

Other supplementary codes are also provided in classifications, for operational purposes, to facilitate the coding of responses to:

Inadequately described, where a response contains insufficient information to be coded to any level of the classification;

Not stated, where no response is provided; and

Not applicable, where the question does not apply to the person and so no response is required (for example, Year of Arrival in Australia is not applicable for people born in Australia).